With rapid advance of alternative energy industry, such as wind power generation, the demand for large-scale, multi-axis/multi-process machine tools is increasing therewith. Consequently, the demand for rotary spindle heads used in those machine tools that are capable of withstanding high torque and high rotation speed are increasing as well.
Please refer to FIG. 1, which shows a conventional large-scale multi-axis machine tool. As shown in FIG. 1, a working bench 82 that is mounted on a first rail set 81 is disposed on a platform 20 for a workpiece to be placed thereon, and there is a frame 83 having a second rail set 84 mounted thereon that is disposed above the platform 80 while providing a third rail set 85 to mounted on the second rail set 84 whereas a spindle head 9 coupled to a seat 86 is mounted on the third rail set 85, by that a system capable of moving in a space defined by a three dimensional XYZ axis model can be constructed since the first, the second and the third rail sets 81, 84, 85 are disposed perpendicular to each other.
Moreover, the spindle head 9 is composed of a spindle unit 91 and a fork-shaped spindle base 92, whereas the spindle unit 91 is further composed of: a rotary base 911, a spindle 912 and a cutting tool 913 that is mounted on the spindle 912 so as to be brought along to rotate with the rotation of the spindle 912, in that the rotary base 911 is hinged to the two supporting arms 921, 922 of the fork-shaped spindle base 92 for enabling the rotary base 911 to perform an axial rotation indicated by the arrow A shown in FIG. 1. Consequently, in addition to that the spindle head 9 is mounted on the seat 86 in a manner that it is able to perform another axial rotation indicated by the arrow C shown in FIG. 1, a multi-axis machine tool is constructed.
Generally, there are three types of spindle head design, which are a design of a spindle head to be driven by serve motors coupled to a speed reduction mechanism, a design of a spindle head to be driven directly by torque motors, and a design of a spindle head to be driven by torque motors coupled to a speed reduction mechanism.
There are examples relating to the design of a spindle head to be driven by serve motors coupled to a speed reduction mechanism that are disclosed in U.S. Pat. No. 5,257,883, U.S. Pat. No. 5,996,329, etc, in which the disclosure of the U.S. Pat. No. 5,257,883 shows a fork-shaped structure with two supporting arms that is provides for enabling a spindle head to be driven by the rotation of a bevel gear reduction system as the gear system is driven by a servo motor; and the disclosure of the U.S. Pat. No. 5,996,329 uses two serve motors that are coupled respectively to a reduction mechanism of worm gears and worm wheels and another bevel gear reduction system for controlling the rotation of a spindle in two different rotation directions that are orthogonal to each other.
Since the output torque of the serve motor used for driving the spindle head to rotate is comparatively smaller, the speed reduction mechanism coupled to the servo motor must be designed with high reduction ratio from several tens-to-one to substantially several hundreds-to-one so as to enlarge the output torque of the servo motor sufficiently enough for the rotary spindle head to operate. However, the foresaid design using serve motors coupled to a speed reduction mechanism of high reduction ratio is short in that: (a) the mechanical transmission of high reduction ratio can cause considerable loss in mechanical efficiency in comparison to other transmission means; (b) the mechanical transmission of high reduction ratio will greatly reduce the rotation speed of the spindle head so that it can not be used for high speed machining; and (c) the mechanical transmission of high reduction ratio usually is designed with larger volume that the design degree-of-freedom of the rotary spindle head can be adversely affected.
One example relating to the design of a spindle head to be driven by directly by torque motors is disclosed in U.S. Pat. No. 7,293,340, in which there are two torque motors disposed respectively at the two supporting arms of a fork-shaped structure so as to be used for directly and cooperatively driving a spindle head to rotate.
Since torque motor is able to output comparatively higher torque, the design of a spindle head to be driven by directly by torque motors is featuring by its low power loss and high rotation speed. Nevertheless, in addition to the cost increasing from the use of two torque motors, in a design point of view, when larger torque motors with larger torque outputs are chosen for conforming to the operation requirement of its spindle head, the overall volume as well as cost will increase with the increasing of its torque motor size and thus the design flexibility is compromised.
One example relating to the design of a spindle head to be driven by torque motors coupled to a speed reduction mechanism is disclosed in U.S. Pat. No. 7,470,095, in which a spindle head that is hinged inside a fork-shaped spindle base is brought to rotate by the use of a torque motor through a first-order gear transmission mechanism, and thereby, the torque output of the spindle head can be increased.
However, in U.S. Pat. No. 7,470,095, not only the torque motors are not disposed coaxially to the rotation axis of the spindle head, but also the selection of the torque motors along with the transmission mechanism that are capable of being used in this disclosure are restricted by the limited space available, so that the output torque of the spindle head resulting from the aforesaid disclosure is only ideal for light and medium machining applications. Moreover, since its input shaft and output shaft are not coaxially disposed, the dynamic balance of this disclosure is not satisfactory that requires to be improved.